You can install the Ethernet kext manually :
Remove any previous failed attempts from /Library/Extensions and /System/Library/Extensions.
Open Finder and browse to /Library/Preferences/SystemConfiguration/ then delete these two files :
- NetworkInterfaces.plist
- preferences.plist
Download this
kext and put it on your desktop.
Download and run
KextBeast selecting /Library/Extensions as the location.
In Terminal :
Press return, type your password (it will not show on screen), press return again and when the command has completed, restart.